You can use SSI to (for example) include a common header and footer in your pages, so you don't have to repeat code as much, and changing one included file updates all of your pages at once. You just put it in your HTML page as per normal.

SHTML is a file extension that lets the web server know the file should be processed as using Server Side Includes (SSI). (HTML is...you know what it is, and DHTML is Microsoft's name for Javascript+HTML+CSS or something).

SHTML stands for Server-Side Include Hypertext Markup LanguageSSI lets you embed a number of special 'commands' into the HTML itself. When the server reads an SSI document, it looks for these commands and performs the necessary action. For example, there is an SSI command which inserts the document's last modification time. When the server reads a file with this command in, it replaces the command with the appropriate time.

SHTML is a file extension that lets the web server know the file should be processed as using Server Side Includes (SSI). (HTML is...you know what it is, and DHTML is Microsoft's name for Javascript+HTML+CSS or something).

It’s just HTML with Server Side Includes.The SHTML files are written in the HTML language and are inserted into an HTML website. These files are then processed by a Web server before being displayed on a user's Web browser.